This application claims the priority of German Application No. 196 25 004.8, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a triggering circuit for a passenger restraint system for motor vehicles and, more particularly, to a triggering circuit for a passenger restraint system for motor vehicles with at least one belt tightener for tightening a safety belt associated with a vehicle seat. The triggering circuit evaluates the acceleration signal from an accelerometer and delivers a triggering signal to the belt tightener when a critical collision is detected.
A triggering circuit for a passenger restraint system, especially a driver airbag, is known from German Patent document DE 39 42 011 A1 (corresponding to U.S. Pat. No. 5,157,268, the disclosure of which is expressly incorporated by reference herein). In this system, the electrical acceleration signal of an accelerometer is integrated several times using a special integration method and results in the triggering of the restraint system provided certain thresholds are simultaneously exceeded by the various integrals. This triggering circuit is designed exclusively for triggering the restraint system in the event of a front-end collision. Although the triggering circuit has an evaluation channel for detecting a rear-end collision, this channel serves to suppress improper triggering of the restraint system in the event of a rear-end collision. A threshold for the integral of the inverting acceleration signal is set for detecting a rear-end collision.
In addition, a safety belt system with a locking device for locking the safety belt when the motor vehicle is involved in a front-end or rear-end crash is known from German Patent document DE 44 28 921 A1. The locking device is controlled by a crash detector, which is not specified any further, to detect a front-end or rear-end crash. The function of the crash detector, especially the types of triggering criteria in a rear-end crash, are not disclosed in this reference.
In a rear-end collision, the vehicle occupants are endangered primarily by two effects: in the ramp effect, the passenger slides upward along the seat back following a rear-end collision and can strike the roof with his/her head. In the rebound effect the passenger is flung forward after being pressed into the seat back and can strike the steering wheel with his/her head. The risk of injury in a rear-end collision can be reduced by triggering the belt tightener, provided the belt tightener is triggered at the correct point in time. This requirement is more stringent for the case when the rear-end collision is followed by a front-end collision that triggers the airbag.
The goal of the invention is to provide an evaluation channel with a suitable method for a triggering circuit, by which channel a critical rear-end collision can be reliably detected and the belt tightener triggered at a point in time at which an optimum protective effect can be achieved.
This goal is achieved in the triggering circuit for a passenger restraint system for motor vehicles with at least one belt tightener for tightening a safety belt associated with a vehicle seat. The triggering circuit evaluates the acceleration signal from an accelerometer and delivers a triggering signal to the belt tightener when a critical collision is detected. The triggering circuit comprises an,evaluation channel for detecting a rear-end collision. The channel includes a backward displacement determination stage with which a travel signal that characterizes the backward displacement of a passenger can be determined on the basis of the acceleration signal that appears in the event of a rear-end collision. The channel also includes a travel threshold value switch connected downstream of the backward displacement determination stage for the travel signal that sets a high signal when a backward displacement threshold value is exceeded by the travel signal. The presence of this high signal is necessary in the event of a rear-end collision for the triggering signal to be output to the belt tightener. In the method for use with the triggering circuit, a travel signal is determined that characterizes the backward displacement of the passenger by means of two integrations from an acceleration signal that occurs in the event of a rear-end collision. The travel signal is compared with a backward displacement threshold value. The triggering signal is output to the belt tightener when threshold value is exceeded.
The advantages of the invention over the prior art consist in the fact that the triggering point in time is not linked to abstract thresholds for accelerations or speeds, but rather the rearward displacement of the passenger in a rear-end collision, calculated from the acceleration signal by double integration, is used as the required triggering criterion. Rearward displacement describes the change in position of the passenger during the collision and is therefore the primary parameter for evaluating the risk to the vehicle passenger. In addition, rearward displacement is a graphic parameter that can be read directly from films of rear-end collision tests.
Although it is known from the literature, especially the above-mentioned German Patent document DE 39 42 011 A1, to use the forward displacement of the passenger in a front-end collision calculated from the acceleration signal by double integration as the criterion for triggering for an airbag, this has not worked well in practice. The long time required for the airbag to deploy means that in a front-end collision the triggering decision must be made very early so that the forward-displacement value that is calculated is still insufficiently accurate to be able to couple the triggering decision to it. This problem is addressed in German Patent document DE 43 30 486 A1 (corresponding to U.S. Pat. No. 5,445,413, the disclosure of which is expressly incorporated by reference herein), for example. Therefore, in the triggering circuits that are predominantly in use today, forward displacement of the passenger is not used to determine the triggering point in time.
However, the present invention recognizes that other conditions prevail in a rear-end collision. From films of rear-end collision tests, a rearward displacement of approximately 400 mm has been determined to be the optimum triggering criterion. With this backward displacement, the backward displacement movement of the passenger has reached its reversal point, so that the belt of the passenger has been deployed to the maximum extent and, after the belt tightener is triggered, its tightening distance can be utilized to the maximum. In the following phase involving rebounding toward the steering wheel, the passenger is restrained as soon as possible by the tightened belt. With the above-mentioned triggering criterion, depending on the speed of impact, triggering times in the range from 80 ms to 120 ms have been found.
Since the time between triggering and completion of the tightening process is comparatively short at approximately 7 ms, a rearward displacement threshold value of 400 mm can be used for the backward displacement movement modeled in the triggering circuit as a triggering criterion without deduction. At the triggering point in time, the travel signal derived from the acceleration signal by double integration is at its highest pitch for backward displacement, so that good accuracy and reproducibility of the triggering point is achieved. Using the triggering criterion thus generated, triggering of the belt tightener occurs only in a rear-end collision at a speed of more than 15 km/h. At a lower impact speed, the travel signal that is generated does not reach the set backward-displacement threshold value of 400 mm.
In one advantageous improvement, the exceeding of a speed threshold value by the speed signal derived by simple integration of the acceleration signal is used as a triggering criterion that must simultaneously be met with the rearward displacement threshold value. This signal characterizes the increase in the weight of the vehicle that takes place in a rear-end collision and is therefore a measure of the severity of the rear-end collision. The increase in speed reaches its end value, specified by the principle of linear momentum for the inelastic collision for average impact speeds of approximately 30 km/h at approximately 80 msec and even earlier at higher impact speeds. In this way, the maximum speed increase is reached for the case in which triggering is desired at a point in time at which the backward displacement threshold value is exceeded by the travel signal. Therefore, the severity of the rear-end collision that causes triggering is determined exactly and reproducibly using the threshold value that the speed signal must exceed if triggering is to occur. This permits a sharp delimitation from a rear-end collision with a low impact energy. The exact value of the speed threshold is determined from films of rear-end collision tests and the damage done to the test vehicles.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.